I would like to join the Affiliate program of Amazon and I am wondering, if the comissions are paid only for the products sold and shipped by Amazon itself or if I can "link" also to the products of the "hosted" vendors (like: "Sold and shipped by xxx")?
Yes, you get advertising fees for third party (as they now call them) products.
Yes, you get advertising fees for marketplace products.
For other Amazon sites
As every Amazon site (per country) has an own affiliate program, it’s theoretically possible that terms differ.